1. Field of the Invention
The present invention relates to a communication system switching method. The present invention particularly relates to the communication system switching method of switching from a communication system to another different communication system, and a terminal device using the method.
2. Description of the Related Art
Recently, cdma2000 1x-EV DO (hereinafter referred to as EV-DO) has been developed as a next-generation high-rate radio communication system. The EV-DO system is a new version, which is specialized for the purpose of data communication and increased in transmission rate, of the cdma2000 1x system, which is an extended version (third generation system) of the cdmaOne system. “EV” means evolution, and “DO” means data only.
The EV-DO system is approximately the same as the cdma2000 1x system in the configuration of a radio interface of an uplink channel from a radio communication terminal to a base station. As for the configuration of the radio interface of a downlink channel from a base station to a radio communication terminal, whereas the bandwidth of 1.23 MHz is the same with that of the cdma2000 1x system, the modulation method and the multiplexing method are much different from those of the cdma2000 1x system. As for the modulation method, whereas QPSK and HPSK are used in the cdma2000 1x system, QPSK, 8-PSK, or 16QAM is selectively used in the EV-DO system according to a radio receive state of a downlink channel of a radio communication terminal. As a result, a high transmission rate with low error resistance is used when the radio receive state is good, and a low transmission rate with high error resistance is used when the radio receive state is bad.
As for the multiplexing method for enabling multiple communications from one base station to a plurality of radio communication terminals, TDMA (Time Division Multiple Access) is employed in which communications with a plurality of radio communication terminals are performed in such a manner that time is divided in units of 1/600 second, a communication is performed with one radio communication terminal in each unit time, and the radio communication terminal to be communicated is switched every unit time, rather than CDMA (Code Division Multiple Access) which is employed in the cdmaOne system and the cdma2000 1x system.
A radio communication terminal measures a carrier-to-interference power ratio (hereinafter abbreviated as CIR) of a pilot signal as an index of a radio receive state of a downlink channel from a base station to be communicated, predicts a radio receive state during the next reception time slot on the basis of a variation of the CIR, and notify “a maximum transmission rate which enables to receive with a error ratio that is lower than a predetermined rate”, which is expected from the predicted radio receive state to the base station as data rate control bits (hereinafter referred to as DRC)predetermined. The predetermined error rate is usually set to about 1% though it depends on the system design. The base station receives the DRCs from a plurality of radio communication terminals, and a scheduler function in the base station determines with which radio communication terminal is to communicate in each division unit time. Basically, as high a transmission rate as possible is decided on the basis of a DRC sent from each radio communication terminal and is used for a communication with it.
With the above configuration, the EV-DO system enables a maximum transmission rate of 2.4 Mbps (mega-bits per second) per sector in a downlink channel. This transmission rate is the sum of amounts of data communications from one base station to a plurality of radio communication terminals in one frequency band and in one of a plurality of sectors (usually, a plurality of sectors exist). The transmission rate increases if a plurality of frequency bands is used.
JP-A-2002-300644 is referred to as a related art.
In the case of that the radio communication terminal can employ not only the EV-DO system but also another radio communication system, such as a wireless LAN system (hereinafter “W-LAN”) or a personal handyphone system, and one of these radio communication systems is selected to employ actually. When the radio communication terminal is used, a criterion used to select a radio communication system to be used is important in order to satisfy the general user request for a “stable communication at a high transmission rate”. Especially, in the case that the W-LAN, the EV-DO system or the personal handyphone system is available in the radio communication terminal, and besides, the maximum transmission rates greatly differ by about 100 times, e.g., from 11 Mbps of the W-LAN based on IEEE802.11b to 128 kbps of the personal handyphone system, the actual transmission rate will be greatly varied in accordance with the difference of switching methods. In order to meet the above request under this condition, the criterion for switching must be changed between when the transmission rate of the currently employed communication system is higher than the transmission rate of another communication system to be selected, and when the transmission rate of the current system is lower than the transmission rate of another communication system to be selected.
Since the transmission characteristic of a radio communication system greatly depends on the quality of a radio transmission path (hereinafter “transmission path quality”) in general, the switching between the radio communication systems is determined based on the transmission path quality. However, when radio communication systems to be selected include a system, such as the EV-DO system, in which the transmission rate, the communication method and the transmission output differ between the uplink line and the downlink line, it is preferable that the transmission path qualities of the uplink line and the downlink line be measured independently. On the other hand, the radio communication terminal cannot measure the transmission path quality of the uplink line while the radio communication terminal can measure the transmission path quality of the downlink line. In addition, unless a new control signal is used in the radio communication system in consideration of easiness of feasibility, the transmission path quality of the uplink line is not notified from the base station. Therefore, it is more difficult to obtain the transmission path quality of the uplink line. Further, when the terminal device travels between the areas covered by different radio communication systems, it is preferable, in consideration of the stabilization of communication, that the radio communication systems are not be switched within a short period of time, and that hysteresis is provided for the switching control.